This invention pertains generally to an apparatus comprising a reactor for thermal oxidation of silicon wafers at high temperatures and high pressures, in controlled atmospheres, and for other processes of thermal treatment requiring silicon wafers or other work pieces to be held under such conditions. This invention pertains particularly to improvements relating to the reactor and a closure for the reactor.
Thermal oxidation of silicon wafers at high temperatures and high pressures, in controlled atmospheres, is discussed in R. J. Zeto, N. O. Korolkoff, and S. Marshall, "Pressure Oxidation of Silicon: An Emerging Technology", Solid State Technology, Vol. 22, No. 7, pp. 62-69, July 1979. As discussed therein, such oxidation may be practiced in an atmosphere of steam, which may be formed by pyrogenic reaction of hydrogen and oxygen.
Various apparatus, which may be used for thermal oxidation of silicon wafers at high temperatures and high pressures in a controlled atmosphere of steam, are disclosed in Japanese (Laid-Open) Patent Application No. 1975-114,179 of Mitsubishi Denki Kabushiki Kaisha; Japanese (Laid-Open) Patent Application No. 1977-154,360 of Tel-Thermco Engineering Co., Ltd.; U.S. Pat. No. 4,018,184 of Mitsubishi Denki Kabushiki Kaisha; U.S. Pat. No. 4,154,192 of Mitsubishi Denki Kabushiki Kaisha; U.S. Pat. No. 4,167,915 of Atomel Corporation; N. Tsuboushi, H. Miyoshi, A. Nishimoto, and H. Abe, "Oxidation of Silicon In High-Pressure Steam", Japan J. Appl. Phys., Vol. 16, No. 5, pp. 855-856, 1977; and N. Tsubouchi, H. Miyoshi, A. Nishimoto, H. Abe, and R. Satoh, "High Pressure Steam Apparatus for Oxidation of Silicon", Japan J. Appl. Phys., Vol. 16, No. 6, pp. 1055-1056, 1977.
Such apparatus are characterized by a reactor, which is made of quartz, silicon carbide, or other refractory material of high purity and mechanical strength, and of chemical compatibility, and which is adapted to receive silicon wafers to be oxidized, and a pressure vessel, within which the reactor is disposed. A heating means is disposed within the pressure vessel, near but outside the reactor so as to heat contents of the reactor, but so as to be isolated by the reactor from silicon wafers within the reactor.
Thermal oxidation of silicon wafers may be practiced in such apparatus at high pressures, typically 10 to 25 atmospheres, or higher pressures. One atmosphere is defined by R. J. Zeto et al., op. cit., as follows: EQU 1atm=1.0332 kg (force)/cm.sup.2.
In such apparatus, the pressure within the reactor and the pressure within the pressure vessel are equalized so as to prevent excessive stress tending to rupture the reactor, as the refractory material of the reactor cannot withstand a large differential between its internal and external pressures. It is known for a small differential, typically about 0.2 atm as computed from internal pressure less external pressure, to be maintained so as to prevent leakage of contaminants into the reactor.
As exemplified in U.S. Pat. No. 4,018,184, such apparatus are known of a type wherein a closure for a workpiece portal, through which silicon wafers are inserted and removed, is made of the same material and biased so as to be pressed against the reactor when attached to the reactor, and so as to allow excessive pressure within the reactor to be relieved through the workpiece portal, and wherein a surrounding margin of the workpiece portal and an adjoining margin of the closure are provided with congruent sealing surfaces enabling a hermetic seal to be effected without an intermediate gasket. It is known for the sealing surfaces to be conical as exemplified in U.S. Pat. No. 4,018,184, spherical, or planar. It also is known for the closure to be mounted within a pressure-vessel head, which is used to seal an access portal of the pressure vessel, so as to enable the closure and the pressure-vessel head to be attached and detached as a subassembly.
In prior apparatus of the type described in the preceding paragraph, precise tolerances must be maintained in the manufacture, assembly, and operation of the apparatus so as to facilitate effective attachment of the closure upon attachment of the pressure-vessel head, and so as to minimize misalignments tending to prevent effective provision of a hermetic seal between the reactor and the closure and tending to cause fragile components to fracture. Particularly, the reactor tends to fracture, if curved sealing surfaces of the reactor and the closure are misaligned so as to impart a bending moment on the reactor when the closure is attached. A major cause of misalignments is a lack of dimensional control in the manufacture of cast refractory members, between which the heating means may be supported, and by which the reactor may be supported. Furthermore, in such apparatus, misalignments can be aggravated by thermal expansion of various components of dissimilar materials.
Accordingly, this invention is addressed to the need, in prior apparatus of said type, for improvements enabling a hermetic seal to be effected despite misalignments between the reactor and the closure and reducing tendencies of fragile components to fracture.